'And I want to admit it… In that bill, there were longer sentences. And most of these people are in prison under state law, but the federal law set a trend. And that was overdone. We were wrong about that.'

The bill pumped nearly $10 billion into prison funding. The U.S. incarceration rate has doubled to 2.2 million inmates over the last two decades.

'The good news is we had the biggest drop in crime history and the first eight-year decline in crime in history,' Clinton continued. 'The bad news is we had a lot of people who were essentially locked up who were minor actors for way too long.'

His wife Hillary Clinton - the Democratic frontrunner in the 2016 election - wants to end what she calls the "incarceration generation," which her husband helped create.